The invention's objective is a process for regulating an asynchronous motor which is energized by fast-acting control or correction elements such as, for example, voltage-impressing pulsed dc/ac converters (or inverters), transistorized dc/ac inverters, current-impressing dc-link inverters, etc. The state of development in this field is described, for example, in:
1. Jotten, R. State of the art for regulated 3-phase drive systems. VDE-Fachberichte, 30, VDE Verlag, Berlin, 1979.
2. Jotten, R. Dynamic behavior of asynchronized motors energized by converters. ETG-Fachtagung, ETG-Fachberichte, No. 5, Oct., 1979.
3. Abbondanti, A. and Brennen, M. Variable speed induction motors use electronic slip calculator based on motor voltages and currents. IEEE-Transactions, vol. IA-11, Sept.-Oct. 1975, No. 5.
4. Venkataram, R. Ramaswami, B and Holtz, J. Electronic analog slip calculator for induction motor drives. IEEE Transactions. Industry Electronics and control instruments. Vol. IEC 1=27, 1980.
Further, the invention is concerned with the rotational speed or torque regulation of asynchronous motors, chiefly those with squirrel cage rotors.
The state of the art reveals two lines of development. The dynamic behavior of the motors is today so well investigated that it is possible, with the inverters now available, to construct control and regulating devices which are equivalent or even superior in their dynamic behavior to rectifier energized dc motors. These arrangements require either a measurement of the rotor's position, in which case an additional automatic matching of the regulator to the rotor resistance, which varies with temperature, is required, or a measurement of the field is necessary. Further, the regulation must be carried out mainly as a regulation of the rotational speed, in which case, it is necessary to employ a tachometer motor. The supplementary accessories consisting of tachometers, rotor-position sensors and also, if need be, devices for carrying out a direct measurement of the field, are very undesirable complications, but they are, however, unavoidable if it is desired to effect control over a large rotational speed range, including crawling rotational speeds, with good dynamics. When, instead of field-measuring, use is made of field simulation, problems arise due to the required integration of measured variables without feedback. In addition, the range encompassing the crawling rotational speeds must be excluded. Also troublesome is that initial conditions are necessary in carrying out an integration.
Of course, the majority of applications require the adjustability of the rotational speed, and the requirements with regard to the dynamic behavior, that is control rate, and with respect to the static accuracy and load-independence of the adjusted speed, are comparatively small. Regulating devices for the stator frequency as a main control variable, or the rotational speed as a main control variable, are known for these cases. The regulation is mainly effected with current-fed inverters. Moreover, the dc-link current, used as a substitute for the value of the stator current, is regulated in a secondary circuit. In the case of speed control, the stator frequency is formed as the sum of the frequency of rotor rotation and a limited slip frequency which is specified by the rotational-speed regulating device, and the slip frequency is allocated to the value of the reference current via a characteristic curve former. In a known setup with no tachometer, there is added to the current controller a superimposed voltage regulating circuit whose actual value forms the rectified motor voltage. The required value of the voltage and the input data for the stator frequency are allotted to each other in accordance with a fixed characteristic curve. In the main, there is no slip limitation with this special arrangement. Overshooting the breakdown (stalling) slip is prevented by limiting the rate of change. The two-last-named processes and arrangements are mentioned as an example for the group of applications in which, with regard to the simplicity of the signal processing and low expenditure for sensors, allowance must be made for rise times of the order of 500 ms and poor damping of the transient effects.